1. Field of the Invention
The present invention relates to a liquid-crystal display device employing a liquid-crystal panel and, more particularly, to a liquid-crystal driving circuit and liquid-crystal driving method for improving the response speed of the liquid crystal.
2. Description of the Related Art
Liquid crystals have the drawback of being unable to respond to rapidly changing moving pictures, because their transmissivity changes according to a cumulative response effect. One method of solving this problem is to improve the response speed of the liquid crystal by increasing the liquid-crystal driving voltage above the normal driving voltage when the gray level changes.
FIG. 72 shows an example of a liquid-crystal driving device that drives a liquid crystal by the above method; details are given in, for example, Japanese Unexamined Patent Application Publication No. 6-189232. Reference numeral 100 in FIG. 72 denotes an A/D conversion circuit, 101 denotes an image memory storing the data for one frame of a picture signal, 102 denotes a comparison circuit that compares the present image data with the image data one frame before and outputs a gray-level change signal, 103 denotes the driving circuit of a liquid-crystal panel, and 104 denotes the liquid-crystal panel.
Next, the operation will be described. The A/D conversion circuit 100 samples the picture signal on a clock having a certain frequency, converts the picture signal to image data in digital form, and outputs the data to the image memory 101 and comparison circuit 102. The image memory 101 delays the input image data by an interval equivalent to one frame of the picture signal, and outputs the delayed data to the comparison circuit 102. The comparison circuit 102 compares the present image data output by the A/D conversion circuit 100 with the image data one frame before output by the image memory 101, and outputs a gray-level change signal, indicating changes in gray level between the two images, to the driving circuit 103, together with the present image data. The driving circuit 103 drives the display pixels of the liquid-crystal panel 104, supplying a higher driving voltage than the normal liquid-crystal driving voltage for pixels in which the gray level has increased, and a lower voltage for pixels in which the gray level has decreased, according to the gray-level change signal.
A problem in the image display device shown in FIG. 72 is that as the number of pixels displayed by the liquid-crystal panel 104 increases, so does the amount of image data written into the image memory 101 for one frame, so the necessary memory size increases. In the image display device described in Japanese Unexamined Patent Application Publication No. 4-204593, one address in the image memory is assigned to four pixels, as shown in FIG. 73, to reduce the size of the image memory 101. The size of the image memory is reduced because the pixel data stored in the image memory are decimated, excluding every other pixel horizontally and vertically; when the image memory is read, the same image data are read for the excluded pixels as for the stored pixel, several times. For example, the data at address 0 are read for pixels (a, B), (b, A), and (b, B).
As described above, the response speed of the liquid crystal can be improved by increasing the liquid-crystal driving voltage above the normal liquid-crystal driving voltage when the gray level changes from the gray level one frame before. Since the liquid-crystal driving voltage is increased or reduced, however, only according to changes in the magnitude relationship between the gray levels, if the gray level increases from the gray level one frame before, the same higher driving voltage than the normal voltage is applied regardless of the size of the increase. Therefore, when the gray level changes only slightly, an overly high voltage is applied to the liquid crystal, causing a degradation of image quality.
If the size of the image memory 101 is reduced by decimation of the image data in the image memory 101 as shown in FIG. 73, the problem described below occurs. FIGS. 74A to 74D illustrate the problem caused by decimation. FIG. 74A shows image data for frame n+1, FIG. 74B shows image data for the image in frame n+1 shown in FIG. 74A after decimation, FIG. 74C shows the image data read by interpolation of the decimated pixel data, and FIG. 74D shows the image data for frame n, one frame before. The image for frame n and the image for frame n+1 are identical, as shown in FIGS. 74A and 74D.
If decimation is carried out as shown in FIG. 74C, the pixel data at (A, a) are read as the pixel data for (B, a) and (B, b), and the pixel data at (A, c) are read as the pixel data for (B, c) and (B, d). Thus pixel data with gray level 50 are read as pixel data for a gray level that is actually 150. Therefore, even though the image has not changed from the frame before, pixels (B, a), (B, b), (B, c), and (B, d) in frame n+1 are driven with a higher driving voltage than the normal voltage.
Thus when decimation is carried out, the voltages for the pixels with decimated pixel data are not controlled accurately, and the image quality is degraded by the application of unnecessary voltages.
The present invention addresses the problem above, with the object of providing a liquid-crystal driving circuit and liquid-crystal driving method capable of accurately controlling the response speed of the liquid crystal in a liquid-crystal display device by appropriately controlling the voltage applied to the liquid crystal.
Another object is to provide a liquid-crystal driving circuit and liquid-crystal driving method capable of accurately controlling the voltage applied to the liquid crystal, even if the capacity of the frame memory for reading the image one frame before is reduced.
The present invention provides a liquid-crystal driving circuit that generates image data from gray-scale values of an input image made up of a series of frames. The image data determine voltages that are applied to a liquid crystal to display the input image.
A first liquid-crystal driving circuit according to the present invention includes:
an encoding unit for encoding a present image corresponding to a frame of the input image and outputting an encoded image corresponding to the present image;
a first decoding unit for decoding the encoded image and outputting a first decoded image corresponding to the present image;
a delay unit for delaying the encoded image for an interval corresponding to one frame;
a second decoding unit for decoding the delayed encoded image and outputting a second decoded image;
a compensation data generator for generating compensation data for adjusting the gray-scale values in the present image according to the first decoded image and the second decoded image; and
a compensation unit for generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
The compensation data generator may include:
a data conversion unit for reducing the number of bits with which the gray-scale values of the first decoded image and the second decoded image are quantized, thereby generating a third decoded image corresponding to the first decoded image and a fourth decoded image corresponding to the second decoded image; and
a unit for outputting the compensation data according to the third decoded image and the fourth decoded image.
Alternatively, the compensation data generator may include:
a data conversion unit for reducing the number of bits with which the gray-scale values of the first decoded image or the second decoded image are quantized, thereby generating either a third decoded image corresponding to the first decoded image or a fourth decoded image corresponding to the second decoded image; and
a unit for outputting the compensation data according to the third decoded image and the second decoded image, or according to the first decoded image and the fourth decoded image.
The compensation data generator may also include:
an error decision unit for detecting differences between the first decoded image and the present image; and
a limiting unit for limiting the compensation data according to the detected differences.
The compensation data generator may also include:
an error decision unit for detecting differences between the first decoded image and the present image;
a data correction unit for adding the detected differences to the first decoded image and the second decoded image, thereby generating a fifth decoded image corresponding to the first decoded image and a sixth decoded image corresponding to the second decoded image; and
a unit for using the fifth decoded image and the sixth decoded image to output the compensation data.
Alternatively, the compensation data generator may include:
an error decision unit for detecting differences between the first decoded image and the present image;
a data correction unit for adding the detected differences to the first decoded image or the second decoded image, thereby generating either a fifth decoded image corresponding to the first decoded image or a sixth decoded image corresponding to the second decoded image; and
a unit for outputting the compensation data according to the fifth decoded image and the second decoded image, or according to the first decoded image and the sixth decoded image.
The first liquid-crystal driving circuit may also include band-limiting unit for limiting a predetermined frequency component included in the present image, the encoding unit encoding the output of the band-limiting unit.
The first liquid-crystal driving circuit may also include a color-space transformation unit for outputting luminance and chrominance signals of the present image, the encoding unit encoding the luminance and chrominance signals.
A second liquid-crystal driving circuit according to the present invention includes:
a data conversion unit for reducing a present image corresponding to a frame of the input image to a smaller number of bits by reducing the number of bits with which the gray-scale values of the present image are quantized, thereby outputting a first image corresponding to the present image;
a delay unit for delaying the first image for an interval corresponding to one frame and outputting a second image;
a compensation data generator for generating compensation data for adjusting the gray-scale values in the present image according to the first image and the second image; and
a compensation unit for generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
A third liquid-crystal driving circuit according to the present invention includes:
an encoding unit for encoding a present image corresponding to a frame of the input image and outputting a first encoded image corresponding to the present image;
a delay unit for delaying the first encoded image for an interval corresponding to one frame and outputting a second encoded image;
a decoding unit for decoding the second encoded image and outputting a decoded image corresponding to the input image one frame before the present image;
a compensation data generator for generating compensation data for adjusting the gray-scale values in the present image according to the present image and the decoded image; and
a compensation unit for generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
The compensation data generator may also include a limiting unit for setting the value of the compensation data to zero when the first encoded image and the second encoded image are identical.
A fourth liquid-crystal driving circuit according to the present invention includes:
an encoding unit for encoding the image data generated for a frame of the input image one frame before a present image in the series of frames, and outputting an encoded image;
a first decoding unit for decoding the encoded image and outputting a first decoded image;
a delay unit for delaying the encoded image for an interval corresponding to one frame;
a second decoding unit for decoding the delayed encoded image, and outputting a second decoded image;
a compensation data generator for generating compensation data for adjusting the gray-scale values in the image according to the first decoded image and the second decoded image; and
a compensation unit for generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
The present invention also provides a method of driving a liquid crystal by generating image data from gray-scale values of an image made up of a series of frames, and applying voltages to the liquid crystal according to the image data.
A first method of driving a liquid crystal according to the present invention includes:
encoding a present image corresponding to a frame of the image, thereby generating an encoded image corresponding to the present image;
decoding the encoded image, thereby generating a first decoded image corresponding to the present image;
delaying the encoded image for an interval corresponding to one frame;
decoding the delayed encoded image, thereby generating a second decoded image;
generating compensation data for adjusting the gray-scale values in the present image according to the first decoded image and the second decoded image; and
generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
Generating the compensation data may include:
reducing the number of bits with which the gray-scale values of the first decoded image and the second decoded image are quantized, thereby generating a third decoded image corresponding to the first decoded image and a fourth decoded image corresponding to the second decoded image; and
outputting the compensation data according to the third decoded image and the fourth decoded image.
Alternatively, generating the compensation data may include:
reducing the number of bits with which the gray-scale values of the first decoded image or the second decoded image are quantized, thereby generating either a third decoded image corresponding to the first decoded image or a fourth decoded image corresponding to the second decoded image; and
outputting the compensation data according to the third decoded image and the second decoded image, or according to the first decoded image and the fourth decoded image.
Generating the compensation data may also include limiting the compensation data according to differences between the first decoded image and the present image.
Generating the compensation data may also include:
adding differences between the first decoded image and the present image to the first decoded image and the second decoded image, thereby generating a fifth decoded image corresponding to the first decoded image and a sixth decoded image corresponding to the second decoded image; and
using the fifth decoded image and the sixth decoded image to output the compensation data.
Alternatively, generating the compensation data may include:
adding differences between the first decoded image and the present image to the first decoded image or the second decoded image, thereby generating either a fifth decoded image corresponding to the first decoded image or a sixth decoded image corresponding to the second decoded image; and
outputting the compensation data according to the fifth decoded image and the second decoded image, or according to the first decoded image and the sixth decoded image.
The first method may also include limiting a predetermined frequency component included in the present image, thereby generating a band-limited image, which is encoded to generate the encoded image.
Encoding the present image may include encoding luminance and chrominance signals of the present image.
A second method of driving a liquid crystal according to the present invention includes:
reducing a present image corresponding to a frame of the input image to a smaller number of bits by reducing the number of bits with which the gray-scale values of the present image are quantized, thereby outputting a first image corresponding to the present image;
delaying the first image for an interval corresponding to one frame and outputting a second image;
generating compensation data for adjusting the gray-scale values in the present image according to the first image and the second image; and
generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
A third method of driving a liquid crystal according to the present invention includes:
encoding a present image corresponding to a frame of the input image and outputting a first encoded image corresponding to the present image;
delaying the first encoded image for an interval corresponding to one frame and outputting a second encoded image;
decoding the second encoded image and outputting a decoded image corresponding to the image one frame before the present image;
generating compensation data for adjusting the gray-scale values in the present image according to the present image and the decoded image; and
generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
Generating the compensation data may include setting the value of the compensation data to zero when the first encoded image and the second encoded image are identical.
A fourth method of driving a liquid crystal according to the present invention includes:
encoding the image data generated for a frame of the input image one frame before a present image in the series of frames, and outputting an encoded image;
decoding the encoded image and outputting a first decoded image;
delaying the encoded image for an interval corresponding to one frame;
decoding the delayed encoded image, and outputting a second decoded image;
generating compensation data for adjusting the gray-scale values in the image according to the first decoded image and the second decoded image; and
generating the image data according to the present image and the compensation data.
The compensation data preferably adjust the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval.
Adjusting the gray-scale values of the present image so that the liquid crystal reaches a transmissivity corresponding to the gray-scale values of the present image within substantially one frame interval enables the response speed of the liquid crystal to be controlled accurately.
By coding the image that is delayed, or by reducing the number of bits with which the gray-scale values of the image are quantized, the present invention reduces the capacity of the frame memory needed to delay the image, and avoids inaccuracies caused by decimation.